The central-southern part of the eastern border of the Sichuan-Yunnan rhombic block provides the research strategy of ‘trade space for time' with an interesting fault system, where the segments have similar focal...The central-southern part of the eastern border of the Sichuan-Yunnan rhombic block provides the research strategy of ‘trade space for time' with an interesting fault system, where the segments have similar focal mechanisms and cover almost continuous spectra of elapse rates. We experiment to study the seismological characteristics of different segments with different elapse rates. We employed the de-clustered earthquake catalog for the calculation of b values for each segment. The analysis revealed that different segments have similar b values,which implies that, although different segments have different periods of earthquake recurrence, the 'natural time' for the whole fault system elapses with a homogeneous pace. We extended the earthquake potential score(EPS)for nowcasting earthquakes to a quasi-EPS(q EPS). It is found that q EPS increases with the increase of elapse rates,albeit for those fault segments whose elapse rates have exceeded 1, q EPS may better reflect the seismic hazard.展开更多
Harvard Centroid Moment Tensor (CMT) solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block (NWSSB), western parts of Central Yunnan sub...Harvard Centroid Moment Tensor (CMT) solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block (NWSSB), western parts of Central Yunnan sub-block (CYSB) and eastern part of CYSB. The characteristics of the mean stress fields in these three regions are obtained by fitting to CMT solutions. The stress state in NWSSB is characterized by its sub-horizontal tensile principal axis of stress (T axis) in roughly N-S direction and west dipping compressive principal axis of stress (P axis); the one in western part of CYSB is characterized by its ENE dipping T axis and sub-horizontal medium principal axis of stress (B axis) in roughly N-S direction; the one in eastern part of CYSB is characterized by its sub-horizontal P axis in roughly NNW-SSE direction and sub-horizontal T axis in roughly WSW-ENE direction. Finite element method simulation clearly shows that the Indian Plate imposes great extrusion on Sichuan-Yunnan rhombic block (SYRB) near Assam massif. The value of the simulated compressive principal stress decreases with the distance from Assam massif. The simulated directions of the T axes in SYRB form annular distribution encir cling Assam. For a homogeneous elastic medium with free boundary conditions on the top and bottom surfaces as well as the displacement boundary conditions derived from the GPS observations on the lateral boundaries, the computation results are consistent with the Harvard CMT solutions in NWSSB and western part of CYSB, while inconsistent with the Harvard CMT solutions in eastern part of CYSB. The inconsistency in eastern part of CYSB can be reduced when it includes inhomogeneous elastic media. The stress states in NWSSB and western part of CYSB revealed by the Harvard CMT solutions are not local, which are mainly controlled by the boundary force on the whole region. On the other hand, the stress state in eastern part of CYSB given by the Harvard CMT solutions is local, which may be affected by local topography, material inhomogeneity, and the drag force underneath.展开更多
The Sichuan-Yunnan Block(SYB)is located at the SE margin of the Qinghai-Tibetan Plateau(TP).Under the influence of the southeastward movement of material originated from the TP,intense crustal deformation,frequent sei...The Sichuan-Yunnan Block(SYB)is located at the SE margin of the Qinghai-Tibetan Plateau(TP).Under the influence of the southeastward movement of material originated from the TP,intense crustal deformation,frequent seismic activity,and complex geological structures are observed in the SYB.The Lijiang-Xiaojinhe fault(LXF)goes through the central part of the SYB,dividing it into two blocks from north to south,and forming an intersecting fault system with the surrounding faults.This paper firstly introduces the morphology and the nature of the LXF,the distribution of the regional surface displacements and the focal mechanisms,and then analyzes the medium deformation and the effects of faults.Moreover,according to the regional tectonics and geophysical patterns,the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone.Since seismic anisotropy is an essential property for detecting crustal stress,deep structures and dynamical mechanisms,this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area.There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB,possibly associated with a clear boundary adjacent to the LXF.Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF,although this boundary is not consistent with the LXF in strike.The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crustmantle decoupling in the south part,even though this conclusion needs to be further verified by more studies.Presently,the scientific understanding of the deep tectonics and the media deformation around the“generalized”LXF i.e.the LXF with the Jinpingshan fault on its eastern side,is still insufficient,and related equivocal topics deserve more in-depth studies.展开更多
Based on the seismic station data sets from Sichuan and Yunnan provinces,we employed a multi-step seismic location method( Hypo2000 + Velest + HypoDD) to precisely locate the 7,787 earthquakes that occurred during 201...Based on the seismic station data sets from Sichuan and Yunnan provinces,we employed a multi-step seismic location method( Hypo2000 + Velest + HypoDD) to precisely locate the 7,787 earthquakes that occurred during 2010-2015 along the eastern boundaries of the Sichuan-Yunnan rhombic block,namely from southern Dawu to the Qiaojia segment.The final results show that location precision is greatly advanced and epicenter distribution exhibits good consistency with the linear distribution of the seismic faults. Earthquake distribution is quite intensive at the intersection region in the southern segment of the Xianshuihe fault,the Anninghe fault zone,the Xiaojinhe fault zone and the Daliangshan fault zone to the east. The depth profile of seismicity shows a clear stepwise activity along the active seismic fault zones. The profile crossing the faults of the Xianshuihe,Anninghe,and Daliangshan presents a complex interaction among faults near the multiple faults intersection region,Shimian,where the earthquakes are obviously divided into two groups in depth. Earthquakes are very rare at the depth of 15km-20 km,which is consistent with the region of the plastic rheology between 14km-19 km calculated by Zhu Ailan et al.,( 2005).展开更多
In the paper, the distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background, especially the relation to Sichuan-Yunnan and Sichuan-Qinghai crustal blocks have b...In the paper, the distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background, especially the relation to Sichuan-Yunnan and Sichuan-Qinghai crustal blocks have been studied. The main results are: a) Strong earthquakes in Sichuan-Yunnan area distribute mainly in Sichuan-Yunnan and Sichuan-Qinghai crustal blocks; b) Most of strong earthquakes of the two blocks distribute mainly along their boundary faults; c) A few strong earthquakes are not obviously related to active faults. It shows that the relation between strong earthquakes and geological tectonics can be very complex; d) There is a certain correlativity for seismic activities among boundary faults of the two blocks, but they have different features; e) There are some anomalous changes of velocity structures in the deep crust of boundary faults of the two blocks. Many boundary faults, especially Longmenshan fault, cut obviously the Moho discontinuity. The Xianshuihe fault, a typical strike-slip fault, has no obvious indication of cutting the Moho discontinuity, but has distinct low-velocity zone in different depths.展开更多
Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes (1994-2005) in four sub-blocks of Sichuan and Yunnan Provinces, namely S...Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes (1994-2005) in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is: EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference (also referred to as consistency parameter θ^- ) between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of θ^- versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in θ^- value and dislocation types of seismogenic faults.展开更多
Based on the concept of "active blocks" and spatial distribution of historical earthquakes with surface ruptures as well as major and subordinate active faults. The Sichuan-Yunnan region can be divided into ...Based on the concept of "active blocks" and spatial distribution of historical earthquakes with surface ruptures as well as major and subordinate active faults. The Sichuan-Yunnan region can be divided into four first-order blocks. They are the Markam block (I), the Sichuan-Yunnan rhombic block (II), Baoshan-Pu'er block (III), and Mizhina-Ximeng block (IV). Cut by sub-ordinate NE-trending active faults, the Sichuan-Yunnan rhombic block (II) can be further divided into two sub-blocks: the northwestern Sichuan sub-block (II1) and the middle Yunnan sub-block (II2), while the Baoshan- Pu'er block (III) can be further divided into three sub-blocks: Baoshan sub-block (III1), Jinggu sub-block (III2), and Mengla sub-block (III3). A quantitative study of offset landforms is carried out and the basic types of active faults and their long-term slip rates along the major boundaries of active blocks of different orders in the Sichuan-Yunnan region are determined, through slip vector analysis, the motion states of the active blocks are clarified and the deformation coordination on the block margins is discussed. It is suggested that the tectonic motion of the blocks in this region is a complex or superimposition of three basic types of motions: southeastward sliding, rotating on vertical axis, and uplifting. The Markam block (I), the northwestern Sichuan sub-block (II1), and middle Yunnan sub-block (II2) have a southeastward horizontal sliding rate of 1-5 mm/a, clockwise rotating angular rate of 1.4-4(/Ma, and uplifting rate of about 1 mm/a. The Baoshan-Pu'er (III) and Mizhina-Ximeng (IV) blocks have also been extensively clockwise rotated. This pattern of motion is a strain response to the collision between the Indian and Eurasian plates and the localized deformation and differential slip on the block margins associated with the northward motion of the Indian Plate. Because a set of transverse thrusts between the blocks absorbs and transforms some components of eastward or southeastward sliding motion, the eastward escape or extrusion of the Tibetan Plateau is limited as "imbricated thrusting transformation-limited extrusion model".展开更多
The Xiaojiang fault is a major active left-lateral fault along the southeastern margin of the Tibetan Plateau.The largest historical earthquake in Yunnan Province, with a magnitude 8 and a mean coseismic left-lateral ...The Xiaojiang fault is a major active left-lateral fault along the southeastern margin of the Tibetan Plateau.The largest historical earthquake in Yunnan Province, with a magnitude 8 and a mean coseismic left-lateral displacement of ~ 6.9 m, occurred on the western branch of the Xiaojiang fault.Studying this fault is important in understanding current deformation and kinematic characteristics of the Tibetan Plateau.Activities and stretches have been well undertaken on the Xiaojiang fault, while paleoseismic research work is always the weak link on this fault.To investigate the paleoseismic history and large earthquake activity of the Xiaojiang fault, we opened a large trench at the northern edge of Caohaizi sag pond on the western branch of the Xiaojiang fault.Six paleoseismic events have been identified, and named E1 through E6 from the oldest to the youngest.Charcoal and woods are abundant, 20 samples were dated to constrain the ages of the paleoseismic events at 40 000–36 300 BC, 35 400–24 800 BC, 9 500 BC–AD 500, AD 390–720, AD 1120–1620 and AD 1750–present.We associate the youngest event E6 with the 1833 M8 earthquake.Events E4, E5 and E6 show a continuous record of the western strand of the Xiaojiang fault in the late Holocene, with a average recurrence interval of 370–480 yr.Large earthquake recurrence in the late Holocene is far less than the recurrence of 2000–4000 yr posed in previous studies.Thus, the seismic hazard on the Xiaojiang fault should be reevaluated.Furthermore, the irregular recurrence of large earthquakes on the Xiaojiang fault and other faults in the Xianshuihe-Xiaojiang system, indicates the uneven southeastward extrusion of the Sichuan-Yunnan block along the southeastern margin of the Tibetan Plateau.展开更多
The southwestern part of the Sichuan Basin(SW-SCB)is adjacent to the eastern Himalayan syntaxis.Affected by the Indo-Eurasian collision and subsequent intrusion of the Indian plate into the Eurasian plate during the C...The southwestern part of the Sichuan Basin(SW-SCB)is adjacent to the eastern Himalayan syntaxis.Affected by the Indo-Eurasian collision and subsequent intrusion of the Indian plate into the Eurasian plate during the Cenozoic,this region is ideal for the study of the tectonic evolution of the intracontinental lithosphere and the dynamic processes of deep and shallow structures.In this study,we applied the receiver function technique to the data obtained from a recently deployed high-density broadband seismic array across the Sichuan Basin and Sichuan-Yunnan block(SCB-SYB).We conducted a multi-frequency and multi-model correction analysis to image the structure of the mantle transition zone beneath this region.The results showed the660-km discontinuity gradually increasing in depth by 10-30 km beneath the western side of the Anninghe-Xiaojiang Fault,suggesting the presence of thermal anomalies caused by the subducted Indian plate from west to east.At the junction of the SCBSYB,the 410-km discontinuity exhibited a slight uplift of 5-10 km,while the 660-km discontinuity showed a significant depression of~30 km over a lateral range of~150 km.Based on previous surface GPS observation and dynamic numerical simulation studies,we suggest that the sharp lateral small-scale topography of this 660-km discontinuity beneath the SW-SCB may have resulted from dripping delamination of the lithosphere within the strain localization area.Furthermore,the aggregation of delaminated lithospheric material at the base of the 660-km discontinuity determines the regional topography of mantle transition zone discontinuities.In this study,we provided seismological evidence for the challenging detection of small-scale intracontinental lithosphere dripping delamination.Moreover,it provides a new view for studying deep and shallow dynamic processes in intracontinental regions with stress concentration resulting from plate/continental subduction and collision.展开更多
基金supported by the National Natural Science Foundation of China (NSFC, grant number U2039207)。
文摘The central-southern part of the eastern border of the Sichuan-Yunnan rhombic block provides the research strategy of ‘trade space for time' with an interesting fault system, where the segments have similar focal mechanisms and cover almost continuous spectra of elapse rates. We experiment to study the seismological characteristics of different segments with different elapse rates. We employed the de-clustered earthquake catalog for the calculation of b values for each segment. The analysis revealed that different segments have similar b values,which implies that, although different segments have different periods of earthquake recurrence, the 'natural time' for the whole fault system elapses with a homogeneous pace. We extended the earthquake potential score(EPS)for nowcasting earthquakes to a quasi-EPS(q EPS). It is found that q EPS increases with the increase of elapse rates,albeit for those fault segments whose elapse rates have exceeded 1, q EPS may better reflect the seismic hazard.
基金Foundation item: State Key Basic research and Development Project from Ministry of Science and Technology of China (2004cb418406)National Natural Science Foundation of China (40521002).
文摘Harvard Centroid Moment Tensor (CMT) solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block (NWSSB), western parts of Central Yunnan sub-block (CYSB) and eastern part of CYSB. The characteristics of the mean stress fields in these three regions are obtained by fitting to CMT solutions. The stress state in NWSSB is characterized by its sub-horizontal tensile principal axis of stress (T axis) in roughly N-S direction and west dipping compressive principal axis of stress (P axis); the one in western part of CYSB is characterized by its ENE dipping T axis and sub-horizontal medium principal axis of stress (B axis) in roughly N-S direction; the one in eastern part of CYSB is characterized by its sub-horizontal P axis in roughly NNW-SSE direction and sub-horizontal T axis in roughly WSW-ENE direction. Finite element method simulation clearly shows that the Indian Plate imposes great extrusion on Sichuan-Yunnan rhombic block (SYRB) near Assam massif. The value of the simulated compressive principal stress decreases with the distance from Assam massif. The simulated directions of the T axes in SYRB form annular distribution encir cling Assam. For a homogeneous elastic medium with free boundary conditions on the top and bottom surfaces as well as the displacement boundary conditions derived from the GPS observations on the lateral boundaries, the computation results are consistent with the Harvard CMT solutions in NWSSB and western part of CYSB, while inconsistent with the Harvard CMT solutions in eastern part of CYSB. The inconsistency in eastern part of CYSB can be reduced when it includes inhomogeneous elastic media. The stress states in NWSSB and western part of CYSB revealed by the Harvard CMT solutions are not local, which are mainly controlled by the boundary force on the whole region. On the other hand, the stress state in eastern part of CYSB given by the Harvard CMT solutions is local, which may be affected by local topography, material inhomogeneity, and the drag force underneath.
基金supported by National Natural Science Foundation of China Projects(42074065)Basic Research Operation Expenses of the Institute of Earthquake Prediction,China Earthquake Administration(2021IEF0103)。
文摘The Sichuan-Yunnan Block(SYB)is located at the SE margin of the Qinghai-Tibetan Plateau(TP).Under the influence of the southeastward movement of material originated from the TP,intense crustal deformation,frequent seismic activity,and complex geological structures are observed in the SYB.The Lijiang-Xiaojinhe fault(LXF)goes through the central part of the SYB,dividing it into two blocks from north to south,and forming an intersecting fault system with the surrounding faults.This paper firstly introduces the morphology and the nature of the LXF,the distribution of the regional surface displacements and the focal mechanisms,and then analyzes the medium deformation and the effects of faults.Moreover,according to the regional tectonics and geophysical patterns,the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone.Since seismic anisotropy is an essential property for detecting crustal stress,deep structures and dynamical mechanisms,this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area.There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB,possibly associated with a clear boundary adjacent to the LXF.Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF,although this boundary is not consistent with the LXF in strike.The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crustmantle decoupling in the south part,even though this conclusion needs to be further verified by more studies.Presently,the scientific understanding of the deep tectonics and the media deformation around the“generalized”LXF i.e.the LXF with the Jinpingshan fault on its eastern side,is still insufficient,and related equivocal topics deserve more in-depth studies.
基金funded by Study on the Optimal Time Window of Single Azimuth Angle,the Three-in-one Project of Earthquake Monitoring,Prediction and Scientific Research of China Earthquake Administration(CEA-JC/3JH-162305)the Special Training Project for Youth Talents for Seismic Network,China Earthquake Administration(20150422)
文摘Based on the seismic station data sets from Sichuan and Yunnan provinces,we employed a multi-step seismic location method( Hypo2000 + Velest + HypoDD) to precisely locate the 7,787 earthquakes that occurred during 2010-2015 along the eastern boundaries of the Sichuan-Yunnan rhombic block,namely from southern Dawu to the Qiaojia segment.The final results show that location precision is greatly advanced and epicenter distribution exhibits good consistency with the linear distribution of the seismic faults. Earthquake distribution is quite intensive at the intersection region in the southern segment of the Xianshuihe fault,the Anninghe fault zone,the Xiaojinhe fault zone and the Daliangshan fault zone to the east. The depth profile of seismicity shows a clear stepwise activity along the active seismic fault zones. The profile crossing the faults of the Xianshuihe,Anninghe,and Daliangshan presents a complex interaction among faults near the multiple faults intersection region,Shimian,where the earthquakes are obviously divided into two groups in depth. Earthquakes are very rare at the depth of 15km-20 km,which is consistent with the region of the plastic rheology between 14km-19 km calculated by Zhu Ailan et al.,( 2005).
文摘In the paper, the distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background, especially the relation to Sichuan-Yunnan and Sichuan-Qinghai crustal blocks have been studied. The main results are: a) Strong earthquakes in Sichuan-Yunnan area distribute mainly in Sichuan-Yunnan and Sichuan-Qinghai crustal blocks; b) Most of strong earthquakes of the two blocks distribute mainly along their boundary faults; c) A few strong earthquakes are not obviously related to active faults. It shows that the relation between strong earthquakes and geological tectonics can be very complex; d) There is a certain correlativity for seismic activities among boundary faults of the two blocks, but they have different features; e) There are some anomalous changes of velocity structures in the deep crust of boundary faults of the two blocks. Many boundary faults, especially Longmenshan fault, cut obviously the Moho discontinuity. The Xianshuihe fault, a typical strike-slip fault, has no obvious indication of cutting the Moho discontinuity, but has distinct low-velocity zone in different depths.
基金National Key Basic Research Development and Programming Project (2004CB418404) and Joint Seismological Science Foundation (105004).
文摘Based on P- and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes (1994-2005) in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is: EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference (also referred to as consistency parameter θ^- ) between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of θ^- versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in θ^- value and dislocation types of seismogenic faults.
文摘Based on the concept of "active blocks" and spatial distribution of historical earthquakes with surface ruptures as well as major and subordinate active faults. The Sichuan-Yunnan region can be divided into four first-order blocks. They are the Markam block (I), the Sichuan-Yunnan rhombic block (II), Baoshan-Pu'er block (III), and Mizhina-Ximeng block (IV). Cut by sub-ordinate NE-trending active faults, the Sichuan-Yunnan rhombic block (II) can be further divided into two sub-blocks: the northwestern Sichuan sub-block (II1) and the middle Yunnan sub-block (II2), while the Baoshan- Pu'er block (III) can be further divided into three sub-blocks: Baoshan sub-block (III1), Jinggu sub-block (III2), and Mengla sub-block (III3). A quantitative study of offset landforms is carried out and the basic types of active faults and their long-term slip rates along the major boundaries of active blocks of different orders in the Sichuan-Yunnan region are determined, through slip vector analysis, the motion states of the active blocks are clarified and the deformation coordination on the block margins is discussed. It is suggested that the tectonic motion of the blocks in this region is a complex or superimposition of three basic types of motions: southeastward sliding, rotating on vertical axis, and uplifting. The Markam block (I), the northwestern Sichuan sub-block (II1), and middle Yunnan sub-block (II2) have a southeastward horizontal sliding rate of 1-5 mm/a, clockwise rotating angular rate of 1.4-4(/Ma, and uplifting rate of about 1 mm/a. The Baoshan-Pu'er (III) and Mizhina-Ximeng (IV) blocks have also been extensively clockwise rotated. This pattern of motion is a strain response to the collision between the Indian and Eurasian plates and the localized deformation and differential slip on the block margins associated with the northward motion of the Indian Plate. Because a set of transverse thrusts between the blocks absorbs and transforms some components of eastward or southeastward sliding motion, the eastward escape or extrusion of the Tibetan Plateau is limited as "imbricated thrusting transformation-limited extrusion model".
基金funded by Natural Science Foundation of China (41372218)Special Foundation of the China Earthquake Administration (Grant No.201108001)+2 种基金Science for Earthquake Resilience of China Earthquake Administration (CEA) (XH13021)Special Projects for Basic Research Work of the Institute of Geology, CEA (IGCEA1418)the program of International S&T Cooperation, Ministry of Science and Technology of China (2015DFA21260)
文摘The Xiaojiang fault is a major active left-lateral fault along the southeastern margin of the Tibetan Plateau.The largest historical earthquake in Yunnan Province, with a magnitude 8 and a mean coseismic left-lateral displacement of ~ 6.9 m, occurred on the western branch of the Xiaojiang fault.Studying this fault is important in understanding current deformation and kinematic characteristics of the Tibetan Plateau.Activities and stretches have been well undertaken on the Xiaojiang fault, while paleoseismic research work is always the weak link on this fault.To investigate the paleoseismic history and large earthquake activity of the Xiaojiang fault, we opened a large trench at the northern edge of Caohaizi sag pond on the western branch of the Xiaojiang fault.Six paleoseismic events have been identified, and named E1 through E6 from the oldest to the youngest.Charcoal and woods are abundant, 20 samples were dated to constrain the ages of the paleoseismic events at 40 000–36 300 BC, 35 400–24 800 BC, 9 500 BC–AD 500, AD 390–720, AD 1120–1620 and AD 1750–present.We associate the youngest event E6 with the 1833 M8 earthquake.Events E4, E5 and E6 show a continuous record of the western strand of the Xiaojiang fault in the late Holocene, with a average recurrence interval of 370–480 yr.Large earthquake recurrence in the late Holocene is far less than the recurrence of 2000–4000 yr posed in previous studies.Thus, the seismic hazard on the Xiaojiang fault should be reevaluated.Furthermore, the irregular recurrence of large earthquakes on the Xiaojiang fault and other faults in the Xianshuihe-Xiaojiang system, indicates the uneven southeastward extrusion of the Sichuan-Yunnan block along the southeastern margin of the Tibetan Plateau.
基金supported by the National Key Research and Development Program of China(Grant No.2017YFC1500302)the National Natural Science Foundation of China(Grant No.42074063)the Key Research Program of the Institute of Geology and Geophysics,Chinese Academy of Sciences(Grant No.IGGCAS-201904)。
文摘The southwestern part of the Sichuan Basin(SW-SCB)is adjacent to the eastern Himalayan syntaxis.Affected by the Indo-Eurasian collision and subsequent intrusion of the Indian plate into the Eurasian plate during the Cenozoic,this region is ideal for the study of the tectonic evolution of the intracontinental lithosphere and the dynamic processes of deep and shallow structures.In this study,we applied the receiver function technique to the data obtained from a recently deployed high-density broadband seismic array across the Sichuan Basin and Sichuan-Yunnan block(SCB-SYB).We conducted a multi-frequency and multi-model correction analysis to image the structure of the mantle transition zone beneath this region.The results showed the660-km discontinuity gradually increasing in depth by 10-30 km beneath the western side of the Anninghe-Xiaojiang Fault,suggesting the presence of thermal anomalies caused by the subducted Indian plate from west to east.At the junction of the SCBSYB,the 410-km discontinuity exhibited a slight uplift of 5-10 km,while the 660-km discontinuity showed a significant depression of~30 km over a lateral range of~150 km.Based on previous surface GPS observation and dynamic numerical simulation studies,we suggest that the sharp lateral small-scale topography of this 660-km discontinuity beneath the SW-SCB may have resulted from dripping delamination of the lithosphere within the strain localization area.Furthermore,the aggregation of delaminated lithospheric material at the base of the 660-km discontinuity determines the regional topography of mantle transition zone discontinuities.In this study,we provided seismological evidence for the challenging detection of small-scale intracontinental lithosphere dripping delamination.Moreover,it provides a new view for studying deep and shallow dynamic processes in intracontinental regions with stress concentration resulting from plate/continental subduction and collision.